The air-fuel ratio in an internal combustion engine affects both engine emissions and performance. With strict modern emissions standards for automobiles, it is necessary to accurately control the air-fuel ratio of the automobile engine, requiring precise measurement of the mass air flow into the engine.
Currently, engine air flow is either measured with a mass air flow sensor or calculated by the speed-density method. Improvements in both types for systems have lead to improved air-fuel ratio control of engines, enabling vehicle manufacturers to meet existing emissions standards.
To achieve yet further improvements on air-fuel ratio control, more engine variables need to be taken into account. An example of a variable that affects emissions output is the amount of REG (re-circulated exhaust gas) in the intake manifold. Mass air flow sensors measure only new air introduced to the manifold and not REG. Suitable sensors that measure REG are not yet available.
System do exist for estimating the amount of REG in a manifold during steady-state conditions based on measured new mass air flow and manifold pressure. However, no system exists for accurately estimating the amount of REG in the manifold during transient (non-steady-state) conditions.
In an ideal engine control system, sensor processing and fuel delivery occur instantaneously to allow precise air-fuel ratio control. In reality, however, it takes a finite amount of time to process sensor measurements to compute proper fueling and a finite amount of time to physically deliver the fuel. The delays in the fuel computation and delivery force the fuel control system to compute the fuel to be delivered in a particular cylinder before the actual delivery of the fuel.
For example, in speed-density systems, air flow estimates are based on measurements of manifold absolute pressure. The aforementioned delays force speed-density systems to read manifold absolute pressure prior to the theoretically optimal time, which would be during the intake event for the cylinder to be fueled. A typical value for this delay is two to three engine events. Because of the dynamic characteristics of engines, the manifold absolute pressure, air flow and REG, can change dramatically between the time manifold absolute pressure is read (and the fuel computed) and the intake event for the cylinder being fueled. Therefore the delay between the calculated air flow and the actual air flow into the cylinder is prominent.
At other times, however, due to parameters such as manifold volume, sensor time constant, etc., the calculated air flow actually leads the actual air flow into the engine. Speed-density calculations are most accurate during static situations. During dynamic situations, when the mass air flow into the engine is changing, the calculated mass air flow into the engine may lead or lag the actual mass air flow. This increases the difficulty of properly controlling the air-fuel ratio during transient conditions.
What is desired is a method of achieving increased accuracy in the determination of proper air-fuel ratio for the vehicle engine in vehicles while taking into account REG in the intake manifold to enable vehicle manufacturers to meet increasingly tightening emissions standards.